The invention relates generally to optical switches and more particularly to techniques for promoting stability in the geometry and the placement of a bubble within an optical switch.
Increasingly, signal transfers within a communications network are carried out using optical signaling, with information being exchanged as modulations of laser-produced light. The equipment for generating and modulating light for optical transmissions is readily available, as are the cables for transmitting the optical signals over extended distances. However, there are concerns with regard to the switching of the optical signals without a significant sacrifice of signal strength.
One technique for switching optical signals is described in U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention. An isolated optical switch that is based on the description in Fouquet et al. is shown in FIG. 1. The optical switch 10 is formed of layers that are patterned on a substrate. The waveguide layers on the substrate include an optional lower cladding layer 14, an optical core 16, and an upper cladding layer, not shown. The optical core may be primarily silicon dioxide, with doping materials that achieve a desired index of refraction. The cladding layers are formed of a material having a refractive index that is significantly different than that of the core material, so that the optical signals are guided along the core. The effective phase index of the waveguide is determined by the refractive indices of the core material and the material of the cladding layers. The layer of core material is patterned into waveguide segments that define a pair of input waveguides 20 and 24 and a pair of output waveguides 22 and 26. After the core material is formed on the lower cladding layer, the upper cladding layer is blanket deposited. A trench 28 is etched into the cladding layers and the core material. A liquid having a refractive index that substantially matches the effective phase index of the waveguides is supplied to the trench. When the liquid is aligned with the waveguides, signals will propagate efficiently through the trench. Thus, signals from the input waveguide 20 will exit from the aligned output waveguide 26, while signals from the input waveguide 24 will exit via the aligned output waveguide 22.
The first input waveguide 20 and the second output waveguide 22 have axes that intersect at or near (preferably near) a sidewall of the trench 28 at an angle of incidence that results in total internal reflection (TIR). When a bubble 30 resides at the intersection of the two axes, the refractive index mismatch creates the TIR condition in which an input signal along the input waveguide 20 is reflected into the second output waveguide 22. However, it should be pointed out that the second input waveguide 24 is not optically coupled to either of the output waveguides 22 and 26, since the misalignment of the optical axes of the waveguides inhibits optical coupling.
The patent to Fouquet et al. describes a number of alternative embodiments for switching the optical switch 10 between a transmissive state and a reflective state. In the transmissive state, the liquid within the trench fills the entire area aligned with the waveguides 20, 22, 24 and 26. One approach to switching between the two states is to include a microheater 38 that controls the formation of a bubble 30 within the liquid-containing trench 28. When the microheater is brought to a temperature that is sufficiently high to form the bubble in the index-matching liquid, the bubble is ideally positioned across the entirety of the interface between each waveguide and the sidewall of the trench. In this ideal situation, only a small quantity of the light leaks into the trench.
The problem with obtaining the ideal condition along the waveguide-to-trench interface is that a bubble is subject to many destabilizing influences. If the surface area covered by a bubble flattened against a trench sidewall is sufficient to fully encompass the lateral extent of the optical fields of the crossing waveguides, such as waveguides 20 and 22 in FIG. 1, the reflection is at a stable maximum. However, any reduction below full lateral extent of the optical fields will cause optical loss. Perhaps more importantly, any variation in the reduced area will cause the reflected optical signal to vary correspondingly. Therefore, any successful approach to confining a bubble within the trench 28 and maintaining the bubble at a sufficiently large size improves the stability of optical reflections, and so improves one important aspect of operational stability of the optical switch 10.
As one approach to providing such operational stability, the electrical power to the microheaters of optical switches may be increased to deliver ample thermal power to create and maintain the bubbles across the entirety of the interface. However, this solution has limited appeal, since the power handling constraints of a large array of optical switches and because of the desirability of operating such an array at the lowest possible power consumption level. Another approach is to appropriately design the shape and size of the trenches holding the bubbles relative to the shapes and sizes of the microheaters which create the bubbles. In the above-identified patent to Fouquet et al., a trench is extended downwardly at opposite sides of the microheater. Thus, V-shaped cuts are etched into a microheater substrate in alignment with the trench. The downward extension of the trench is intended to increase bubble stability by promoting dynamic equilibrium, with fluid boiling at the heaters and condensing at the top of the bubbles. This approach improves stability, but alternative or additional techniques are desired.
Performance stability of an optical switch that has a reflective efficiency based upon the position of a bubble within a liquid-containing trench is enhanced by allowing the liquid to flow from the trench into an adjacent space, while controlling the movement of the bubble relative to either or both of the trench and the adjacent spacing. Surface features are intentionally altered in order to regulate the position of the bubble within the trench. The optical switch includes a transmissive state in which optical signals efficiently propagate from a waveguide into the liquid within the trench, since the liquid and the waveguide have similar refractive indices. The optical switch also has a reflective state in which the optical signals are reflected as a result of the bubble being at the interface of the waveguide with the trench. The adjacent spacing accommodates volume expansion when the bubble is created by activation of a microheater, but the intentionally altered surface features are designed to control the position of the bubble relative to the waveguide-to-trench interface.
The spacing that is adjacent to the trench may be generally perpendicular to the trench. Typically, the spacing is naturally or intentionally formed when a waveguide substrate is connected to a heater substrate. In a switching network, the heater substrate includes at least one microheater for each optical switch in an array of switches. The waveguide substrate includes a liquid-containing trench and two or more waveguides for each optical switch. Coupling of the optical waveguides for a switch depends upon the presence or absence of liquid in alignment with an input waveguide of the switch. As an alternative to forming the adjacent spacing as a result of connecting two substrates, adjacent spacing that accommodates volumetric expansion may be provided by using other techniques, such as layer etching.
In one embodiment, the intentionally altered surface features that control the position of the bubble are raised barriers that partially obstruct the movement or expansion of the bubble into the adjacent spacing. For example, the raised barriers may be partial barriers that are provided by depositing or growing a material, typically a dielectric material, on the heater substrate. In one application, the barriers are positioned within the adjacent spacing on two opposed sides of the microheater, but may include portions which reside within the trench at the other two sides of the microheater. Thus, in addition to providing lateral control of the bubble position, the barrier material may provide longitudinal control along the length of the trench. During the process of fabricating an optical switch, incorporating steps of providing and patterning the barrier material is a relatively small price to pay for the long term reduction (via surface energy variations) or even complete prevention (by physically blocking) of the lateral expansion of a bubble into the spacing that is adjacent to a trench.
In a related embodiment, the surface topography is varied along a target boundary line of contact between the bubble and the structure of the optical switch. Typically, the surface topography is altered along a surface that is formed after the microheater, such as in a dielectric layer that is blanket deposited over the microheater to provide protection of the metallic microheater from chemical attack by the index-matching liquid within the trench. However, the surface topology variations along the target boundary line of contact may be to a layer other than the dielectric layer or to a substrate other than the heater substrate. The change in the surface topography may be used to xe2x80x9cpinxe2x80x9d the bubble along the target contact line. Thus, even if the bubble xe2x80x9cbulgesxe2x80x9d laterally into the spacing between the two substrates, the bubble will be in a proper position against the trench sidewall at the interface between the trench and the input waveguide. The change in surface topography may be local depressions or may be local elevated regions. If the changes are provided by elevating regions, the material could have a low thermal conductivity, so that its lower surface temperature would further inhibit the lateral spread of the bubble.
In a third embodiment, the intentionally altered surface features are provided by removing substrate material from regions of the microheater substrate that correspond to the intended position of the microheater and then depositing a dielectric material within these regions. For example, dielectric material may be formed under or next to opposite sides of the microheater to provide thermal isolation, thereby reducing the loss of heat into the microheater substrate. However, the addition of the dielectric material is reduced in importance if the substrate material is removed so as to provide substantially vertical sidewalls which provide the desired abrupt transitions with regard to heat conductivity. By improving the delivery of heat to the bubbles, the size of the bubble may be increased without an increase in the required power.
In yet another embodiment, auxiliary trenches are formed near the main trench. An auxiliary trench on one or both sides of the main trench may be formed within the waveguide substrate or the heater substrate. The edges of the auxiliary trenches alter the surface energy balance so as to inhibit the lateral expansion or movement of the bubbles beyond the auxiliary trenches. Since the auxiliary trenches are also filled with the index-matching liquid and since the liquid has a lower thermal conductivity than the substrate material (e.g., silica), the auxiliary trenches present a thermal loss barrier from the xe2x80x9chotxe2x80x9d crosspoint of the optical switch. As a result, the auxiliary trench or trenches may help to maintain both the size and the position of the thermally created and maintained bubble.
As a fifth embodiment, either or both of the trench walls and the spacing walls are treated to change the wettability of the walls. Thus, one or both of the surfaces of the waveguide substrate and the heater substrate may have a patterned film that provides an intrinsically different wettability to the index-matching liquid. By varying the surface wettability, the capillary forces acting at the bubble interface will increase. By changing the effective capillary forces in this manner, the stability of the confined bubble can be enhanced.
The desired surface wettability along the walls of the substrate-to-substrate spacing may be obtained by patterning an additional film on one or both of the substrates. As an example, gold may be deposited in the proximity of the trench. Further differences in the wettability relative to the underlying material (e.g., silicon oxide) can be induced by selective self assembly of organic functionalized orthosilicates or chlorosilanes onto the silicon oxide surface or through selective self assembly of alkane thiols onto the gold surface. Specific subgroups of interest are fluorinated long chain hydrocarbon chlorosilane and fluorinated long chain alkane thiol. The surface treatment of the walls of the trench may be designed to promote drying of the waveguide-to-trench interfaces when a bubble is formed, so that liquid residue is less likely to interfere with reflectivity performance. While general treatment of both the trench sidewalls and the substrate surfaces is contemplated, the result would be similar properties among all of the surfaces. However, there are benefits to treating only the spacing surfaces, treating only the trench sidewalls, or treating the spacing surfaces and the trench sidewalls differently.
Another possibility is to alter the surface above the microheater to promote capillary action which more efficiently transfers heat to the liquid in forming the bubble. A metal mesh, sintered metal, or other porous thin layer may be formed atop the heater region to provide capillary holes which draw fluid toward the microheater. Typically, after a bubble is formed, there is a continuous condensation at the top of the bubble and a continuous vaporization along the edges of the microheater. By maintaining a greater portion of the microheater in a wet condition, the optical switch is configured to enhance the ability of providing vaporization as needed, rather than confining vaporization to the edges of the microheater. Consequently, bubble stability is enhanced. Cooling fins may be added to this porous cover layer in order to promote dissipation of the heat when the microheater is deactivated.
One advantage of the invention is that bubble stability is improved. As a result, the optical performance of the optical switch will be improved. Another advantage is that enhanced stability is achieved without increasing the operating power requirements of the optical switch or the switching array in which the optical switch is a member. While additional processing steps are required, the additional steps are neither complex nor costly.